Yes, the US southern border is susceptible to terrorist crossings

From November 2013 to July 2014, officials apprehended 143 individuals listed on the US terror watch list trying to cross the Mexico border and enter the US illegally, according to a confidential Texas Department of Public Safety report obtained by the Houston Chronicle.

And last summer, the US military’s Southern Command warned in an intelligence report that Muslim extremists were using existing migrant smuggling rings in Latin America to gain entry to the US across the Mexico border, according to an account in the Washington Free Beacon.